The protein loses biological activity if the mutation changes an amino acid that is important for protein structure or function. If the protein is an enzyme, it may lose its ability to bind to its substrate or to react with it at the active site. When a changed enzyme can't catalyze a process, certain compounds can build up in the cell until they become poisonous, or essential substances can't be created. A mutation that arises in a major metabolic pathway or is involved in the construction of a cell membrane can be fatal. A genetic illness is defined as a protein deficit that is inherited.